1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device. More particularly, the invention relates to a semiconductor device comprising photo detectors each made of a PN junction diode, as well as to a method of manufacturing such a semiconductor device.
2. Description of the Background Art
As a solid state image sensor for use in video cameras, there exist conventional semiconductor devices using a PN junction diode as a photo detector. FIG. 23 shows a diagram of an equivalent circuit corresponding to one pixel in such a semiconductor device. As shown in FIG. 23, the typical semiconductor device includes per pixel a PN junction diode 10 working as a photo diode, and a control transistor 12 connected serially to the PN junction diode 10.
FIG. 24A is a cross-sectional view of a partial structure corresponding to the equivalent circuit in FIG. 23. FIG. 24B is a plan view of the partial structure corresponding to the equivalent circuit. The conventional semiconductor device comprises a silicon single crystal substrate 14 arranged for use as a P-type semiconductor (simply called the P-type substrate 14 hereunder although including P-type wells formed in an N-type wafer). A surface area of the P-type substrate 14 is divided into single-pixel regions by an isolation oxide film 16.
The P-type substrate 14 is furnished with a gate oxide film 18, a gate electrode 20, and side walls 22 laterally surrounding these elements. On one side of the gate electrode 20 are an N-type region 24 arranged for use as an N-type semiconductor and a P-type region 26 arranged for use as a P-type semiconductor. The N-type region 24 is formed by implanting N-type impurities such as phosphorus (P) or arsenic (As) into the P-type substrate 14 at a predetermined angle relative to the latter, i.e., in such a manner that the N-type impurities reach apart immediately below the gate oxide film 18. After the N-type region 24 is formed, the P-type region 26 is produced by implanting P-type impurities such as boron (B) into the P-type substrate 14 at right angles to the latter. Between the N-type region 24 and the P-type region 26, a PN junction plane 28 is produced.
On the other side of the gate electrode 20 is an LDD (Lightly Doped Drain) structure N-type drain region 30. The N-type drain region 30 is formed by implanting N-type impurities into the P-type substrate 14 in a well-known manner.
In the structure shown in FIGS. 24A and 24B, the N-type region 24 and P-type region 26 constitute a PN junction diode 10 that functions as a photo diode. The gate electrode 20 and N-type drain region 30 make up a control transistor 12 connected to the PN junction diode 10. In operation, light 32 enters the P-type region 26, generating a light signal carrier 34 in the N-type region 24 in a manner proportional to the amount of the incident light 32. The light signal carrier 34 that developed in the N-type region 24 is transferred to the N-type drain region 30 when a predetermined driving voltage is fed to the gate electrode 20.
In the conventional semiconductor device outlined above, an insufficient carrier path that may develop between the PN junction diode 10 and the control transistor 34 prevents the light signal carrier 34 from being adequately transferred from the PN junction diode 10. The result is a so-called afterimage phenomenon. In the structure shown in FIGS. 24A and 24B, a portion formed in a partially submerged manner immediately under the gate oxide film 34 (called the submerged portion hereunder) in the N-type region 24 constitutes the carrier path connecting the PN junction diode 10 to the control transistor 12. To forestall the afterimage phenomenon thus requires providing the submerged portion of the N-type region 24 with a sufficient carrier transfer capability.
In order to confer an adequate carrier transfer capability to the submerged portion of the N-type region 24, it is necessary to implant N-type impurities of a high enough concentration underneath the gate oxide film 18. More specifically, a partially submerged portion containing highly concentrated N-type impurities of a uniform distribution needs to be formed by implanting the impurities into the flat P-type substrate 14 at an angle with respect to the latter.
The trouble is that such a partially submerged portion meeting the above requirements is difficult to form through the use of conventional semiconductor device manufacturing techniques. As a result, conventional semiconductor devices tend to be lacking in the carrier transfer capability of the submerged portions in the N-type region 24 and are thus susceptible to the afterimage phenomenon.
Conventional semiconductor devices offer higher resolutions as the PN junction diode 10 shows a higher sensitivity. The sensitive of the PN junction diode 10 improves as the amount of light incident on the P-type region 26 becomes grater, the area of the PN junction plane 28 becomes wider, and the level of light-gathering efficiency of the P-type region 26 becomes higher.
Conventional semiconductor devices attain higher degrees of integration the narrower the area occupied by the P-type region 26. One known way to enhance the sensitivity of the PN junction diode 10 without increasing the area occupied by the P-type region 26 is by furnishing individual P-type regions 26 with a convex microlens each. The convex microlens condenses diffused light and causes the condensed light to enter the P-type region 26, boosting the sensitivity of the PN junction diode 10. This is an effective technique for maintaining a high degree of integration while attaining a high level of resolution at the same time.
Except for the convex microlens technique, there are few other methods conventionally employed to enhance the sensitivity of the PN junction diode 10 without increasing the area taken up by the P-type region 26. So far, there have not been many in-depth studies on how to enhance the light-gathering efficiency of the P-type region 26 without increasing the area occupied by the PN junction plane 28.